I followed this guide, http://www.podfeet.com/blog/tutorials-5/how-to-configure-verizon-fios-router-to- give-network-control-to-airport-extreme/
Unfortunately, if you follow the steps in this guide, you will wind up with a Double NAT error on the AirPort Extreme.....which can severely restrict connection speeds, and create other issues as well.
"Ignoring" the error....as the guide suggests.....will not make the error go away. So, best to put the guide aside and configure the AirPort correctly.
My advice would be to accept the fact that you will not be able to make the AirPort Extreme the primary router with the Fios Gateway providing your Internet connection. IF....Fios offered a simple modem......not a modem/router or gateway.....with their service.....then....you would be able to set up the AirPort Extreme as the main router for your network.
Unless something has changed within the last few days with Fios, they do not offer a simple modem option to their customers. So, if you still want to try to set up the AirPort Extreme as the main or primary router on your network, I will not be able to assist you, but perhaps someone else can.
I would suggest that you reset the the Fios Gateway back to its original factory default settings, as provided by Fios.
Once you have done that, connect an Ethernet cable from one of the four LAN <--> ports directly to your Mac and check the speed that way to confirm that you are receiving the speed that you are paying for.
Once you have verified this, start over with the AirPort Extreme and reset it back to factory default settings, then set it up again. The setup "wizard" in AirPort Utility will configure the AirPort Extreme in the correct Bridge Mode configuration to work with the Fios Gateway.
Now, connect an Ethernet cable from one of the three LAN <--> ports on the back of the AirPort Extreme directly to the Mac and check the Internet connection speed that way.
IF......the AirPort Extreme is functioning correctly, you should see about the same speed as you measured when the Mac was connected directly to the Ethernet port on the back of the Fios Gateway.
If you don't see about the same speed on the AirPort.....and you have reset the device again a few more times and set it up a few more times.....then the AirPort is defective and will need to be replaced.
There is no reason to even check wireless speeds on the AirPort Extreme until you have verified that Ethernet is working correctly. Another way to say that the same thing is that if Ethernet is not working correctly, then wireless will not be working correctly either.
Please post back on your results.